Cusick (1992) describes Carex nigromarginata as "a beautiful sedge with brightly-colored scales of rich chestnut or purple with a green midrib".

The plants are perennial herbs, 4-12 in. tall, growing in dense tufts, often producing short branches which root. The whole plant often has a flattened appearance as if it had been stepped on. Each stem produces a single inflorescence with these flowering stems of varying height. Some of the culms are short and hidden in the leaves while others are longer and more apparent. The principal leaf blades are very narrow (1.4-4.5 mm wide) and mostly arising from the base. The flowers are unisexual. Staminate flowers occur on the end of the elongated flower structure, the pistillate below. Staminate flowers contain three small, slender, erect stamens which project beyond the petals when pollen is produced. There are 1-4 pistillate flowers (usually 2 or 3) up to .25 in. long, stemless, often crowded, and the lowest sometimes slightly separated but overlapping the next above. The small dry fruit does not open at maturity, is three-angled or nearly round, and contains 3 stigmas. The scales that subtend the pistillate flowers/fruits are often dark red to dark purple sub-marginally. These scales also have a green mid-stripe and pale narrow margins (derived from Gleason 1952 and Crins and Rettig 2002).

This sedge is most easily identified while in fruit. Full ample specimens are useful for identification purposes. These specimens should include the entire plant (i.e. roots, stems, leaves, and fruit).

Carex nigromarginata is a member of section Acrocystis. A few other members of this section are a little similar although C. nigromarginata is really quite a distinctive sedge.

Carex umbellata, C. tonsa var. tonsa, and C. tonsa var. rugosperma should not be confused with C. nigromarginata. The former three all have at least some spikes arising from the base of the culm. These basal spikes can have elongated peduncles and therefore can appear to arise towards the upper parts of the culm. In addition, C. nigromarginata often has some culms that are very short but again these culms have all the spikes arising from the upper portion of the culm.

Carex albicans var. albicans has wider leaves 1.3-2.5(-2.6) mm wide (vs. (1.9-)2.2-3.4(-4.5)mm wide); and most culms are as long as or longer than the leaves (vs. many culms shorter than the leaves). Carex albicans var. emmonsii has the widest leaves (1.4-)1.5-2.5 mm wide; staminate spike peduncle (0.5-)0.7-1.5 mm long (vs. 0.4-0.8 mm long); and perigynia 2.3-3.3 mm (vs. (2.6-)2.8-4.0 mm).